Branched block copolymers and their manufacture

ABSTRACT

Branched block copolymers of from 60 to 95 percent by weight of a monovinyl-aromatic compound and from 40 to 5 percent by weight of a conjugated diene. The copolymers have a structure of the general formula (A1-B-&gt;A2)n-X-(A2&lt;-B)m WHERE THE A&#39;s are non-elastomeric polymer segments based on the monovinyl-aromatic compound, the B&#39;s are elastomeric polymer segments based on the conjugated diene, X is the radical of an at least trifunctional coupling agent and n and m are numbers. The copolymers may be used for the manufacture of highly transparent impact-resistant shaped articles, especially packaging materials.

The present invention relates to branched block copolymers which arebuilt up of a predominant proportion of a monovinylaromatic compound anda lesser proportion of a conjugated diene, and which possess hightransparency, high clarity and good mechanical properties, especially ahigh impact strength.

The manufacture, by polymerization of styrene and butadiene withlithium-hydrocarbons as initiators, of block copolymers in which one ormore non-elastomeric polymer blocks are bonded to one or moreelastomeric polymer blocks, has been disclosed. Depending on the contentof the polymer blocks in the total polymer, these thermoplastic blockcopolymers exhibit non-elastomeric or elastomeric properties. Successivepolymerization of the monomers results in block copolymers having alinear structure. If such linear block copolymers are coupled to oneanother by polyfunctional reactive compounds, branched block copolymershaving a star-shaped structure result. Such branched block copolymers,described, for example, in British Pat. No. 985,614, have a symmetricalstructure and in general exhibit better processability than the linearblock copolymers.

It has also been disclosed that styrene-butadiene block copolymershaving a high styrene content are clear thermoplastics having a highimpact strength. Even though the block copolymers of this type,developed and proposed hitherto, have satisfactory properties in somerespects, there are many practical requirements which they do notfulfil. In particular, their physical and mechanical properties leavesomething to be desired, or the products do not possess the transparencywhich is desirable for many applications.

German Laid-Open Application DOS 1,959,922 discloses branched copolymershaving a star-shaped structure, obtained from a predominant proportionof styrene and a lesser proportion of a conjugated diene, which arestated to combine impact strength, clarity, good processability andresistance to external factors, in one and the same polymer. Thesebranched block copolymers are obtained by coupling styrene-dienetwo-block copolymers in which the terminal polystyrene blocks havedifferent lengths.

It is true that these products exhibit improved properties compared tothe symmetrically branched block copolymers, but they do not prove fullysatisfactory in respect of their mechanical properties, especially theirimpact strength, elongation at break and yield stress.

Unsymmetrical branched block copolymers are also described in GermanLaid-Open Application DOS 2,125,344. The advantage of these copolymers,which possess a homopolymer block in at least one branch, oversymmetrical block copolymers is stated to be the lower solutionviscosity of the polymers. In respect of their mechanical properties(impact strength), the polymers described in DOS 2,125,344, if based ona predominant proportion of styrene, are as unsatisfactory as theproducts known from DOS 1,959,922.

It is an object of the present invention to improve the mechanicalproperties of styrene-butadiene block copolymers which comprise apredominant proportion of styrene, and in particular to provide productshaving an increased impact strength and improved elongation at break. Inaddition, the products should be transparent and as glass-clear aspossible, and should possess good processability.

We have found that this object is achieved and that, surprisingly,non-elastomeric branched block copolymers of a monovinylaromaticcompound and a conjugated diene possessing a quite specific blockcomposition and structure in the branches, exhibit improved propertiesrelative to comparable conventional block copolymers.

Accordingly, the present invention relates to branched block copolymersof from 60 to 95 percent by weight of a monovinyl-aromatic compound andfrom 40 to 5 percent by weight of a conjugated diene of 4 to 8 carbonatoms, which are built up of non-elastomeric polymer segments based onthe monovinyl-aromatic compound and elastomeric polymer segments basedon the conjugated diene and which are manufactured by anionic solutionpolymerization of the monomers by means of a monolithium-hydrocarbon asthe initiator, followed by coupling of the resulting linear blockcopolymer with a polyfunctional coupling agent, wherein the averagestructure of the branched block copolymers corresponds to the generalformula

    (A.sup.1 -B→ A.sup.2).sub.n -X-(A.sup.2 ←B).sub.m

where A¹ and A² are non-elastomeric polymer segments based on themonovinyl-aromatic compound and the B's are elastomeric polymer segmentsbased on the conjugated diene, n and m are numbers, m being equal to orgreater than n and the sum of m and n being at least 3, and X is theradical of the polyfunctional coupling agent by means of which thepolymer blocks, which form the branches, are chemically bonded to oneanother at the polymer segments A², with the provisos that the polymersegment or segments A¹ contains or contain from 50 to 80 percent byweight of the total monovinyl-aromatic compound of the branched blockcopolymer, as copolymerized units, the transition between the polymersegments A¹ and B is sharp and the transition between the polymersegments B and A² is gradual.

Examples of monovinyl-aromatic compounds which can be used to synthesizethe branched block copolymers of the invention are styrene, styreneswhich are alkylated in the side chain, eg. α-methylstyrene andnuclear-substituted styrenes, eg. vinyltoluene or ethylvinylbenzene. Themonovinyl-aromatic compounds may be employed individually or as mixtureswith one another. Preferably, however, styrene alone is used. Examplesof conjugated dienes which can be employed according to the invention,individually or as mixtures with one another, for the manufacture of thebranched block copolymers, are butadiene, isoprene and2,3-dimethylbutadiene. Butadiene and isoprene give particularlyadvantageous results, and of the two butadiene is preferred.

The branched block copolymers of the invention should in total containfrom 60 to 95 percent by weight, especially from 70 to 90 percent byweight, of the monovinyl-aromatic compound and from 40 to 5 percent byweight, preferably from 30 to 10 percent by weight of the conjugateddiene (in each case based on the total monomers employed), ascopolymerized units. The molecular weight of the branched blockcopolymers is as a rule from 100,000 to 1,000,000 and preferably from150,000 to 500,000. These figures relate to the weight average molecularweight, determined by viscosity measurements in toluene at 25° C.

In detail, the branched block copolymers of the invention aremanufactured by successive polymerization of the monomers in solution inthe presence of a monolithium-hydrocarbon as the initiator, withstepwise addition of monomer and of initiator, followed by coupling ofthe resulting living linear block copolymers with a polyfunctionalreactive compound as the coupling agent, as follows:

In a first process stage, the non-elastomeric polymer segment A¹ isproduced by polymerizing a substantial portion of the total amount ofthe monovinyl-aromatic compound by means of a relatively small amount ofthe monolithium-hydrocarbon initiator in an inert solvent underconventional conditions. In this stage, from 50 to 80 percent by weight,preferably from 60 to 78 percent by weight, of the total amount of themonovinyl-aromatic compound employed, overall, for the manufacture ofthe branched block copolymers should be used. The total amount ofmonovinyl-aromatic compound used for the manufacture of the branchedblock copolymers is from 60 to 95 percent by weight, in particular from70 to 90 percent by weight, based on the total monomers used for themanufacture of the polymer.

The amount of the initiator employed in the first stage of the processdepends, above all, on the desired molecular weight of the polymer andis generally from 0.2 to 10 mmoles per mole of the monovinyl-aromaticcompounds employed in the said first process stage. Preferably, from 0.4to 2.5 mmoles of initiator per mole of the monovinyl-aromatic compoundsemployed in the first process stage are used in the said stage. Theinitiators employed are the conventional monolithium-hydrocarbons of thegeneral formula RLi, where R is an aliphatic, cycloaliphatic, aromaticor mixed aliphatic-aromatic hydrocarbon radical, which may be of 1 toabout 12 carbon atoms. Examples of the lithium-hydrocarbon initiators tobe employed according to the invention are methyl-lithium,ethyl-lithium, n-, sec.- and tert.-butyl-lithium, isopropyl-lithium,cyclohexyl-lithium, phenyl-lithium and p-tolyl-lithium. Themonolithium-alkyl compounds where alkyl is of 2 to 6 carbon atoms arepreferred, n-butyl-lithium and sec.-butyl-lithium being particularlypreferred.

The polymerization of the monovinyl-aromatic compounds is carried out insolution in an inert organic hydrocarbon solvent. Suitable hydrocarbonsolvents are aliphatic, cycloaliphatic and aromatic hydrocarbons whichare liquid under the reaction conditions and are preferably of 4 to 12carbon atoms. Examples are isobutane, n-pentane, isooctane,cyclopentane, cyclohexane, cycloheptane, benzene, toluene, the xylenesand others. Mixtures of these solvents may also be employed.Furthermore, the polymerization can be carried out in the presence ofsmall amounts, in general from 10⁻³ to 5 percent by weight, based ontotal solvents, of ethers, eg. tetrahydrofuran, dimethoxyethane, phenylmethyl ether and others, whereby it is possible to influence, in theconventional manner, the rate of polymerization, the configuration ofthe butadiene polymer segment B and the statistical transition betweenthe segments B and A². Preferably, however, no ether is added. Theconcentration of the monomers in the reaction solution is not criticaland can be so chosen that any desired apparatus can be used for thepolymerization. As a rule, the polymerization is carried out in from 10to 30% strength solutions in the inert solvents.

The polymerization is carried out under the conventional conditions foranionic polymerizations with lithium-organic compounds, eg. in an inertgas atmosphere, with exclusion of air and moisture. The polymerizationtemperature may, for example, be from 0° to 120° C and is preferablykept at from 40° to 80° C.

In this first stage of the process, the polymerization is taken tovirtually complete conversion of the monovinylaromatic compoundsemployed. This gives a solution of non-elastomeric, living linearpolymers of the monovinyl-aromatic compounds (polymer segment A¹) withactive terminal lithium-carbon bonds capable of further addition ofmonomers.

In the second stage of the process, the polymer segments B, followed bythe polymer segments A², are polymerized onto the living active chainends of these polymer segments A¹, to form the polymer block (A¹ --B→A²)of the branched structure, and at the same time, in the same reactor,the copolymer blocks (B→A²) of the branched structure are formed. Forthis purpose, a further amount of initiator and a mixture of theremaining monovinyl-aromatic compound and the conjugated diene are addedto the fully polymerized reaction solution from the first stage of theprocess, and polymerization is carried out. The amount of conjugateddiene is from 5 to 40 percent by weight, preferably from 10 to 30percent by weight, of the total monomers employed for the manufacture ofthe branched block copolymers of the invention. The amount of freshinitiator which is added to the reaction solution in the second stage ofthe process should be as great or greater, than the original amount ofinitiator which has been employed in the first stage of thepolymerization process. Preferably, the amount of fresh initiator addedin the second stage of the process is from 1 to 15 times, and inparticular from 1 to 10 times, the amount of initiator added originally.It is particularly advantageous to add from 1 to 5 times the amount,especially when, as explained in more detail below, trifunctional ortetrafunctional coupling agents are employed in the subsequent couplingreaction. Suitable initiators are the monolithium-hydrocarbons, whichcan also be employed in the first stage of the process; preferably, theinitiator used is identical to that used in the first stage of theprocess. It is advantageous to add the fresh initiator to the reactionsolution before the monomer mixture of the remaining monovinyl-aromaticcompound and the conjugated diene is added.

In the second process stage, the same polymerization conditions may bemaintained as in the first stage, and here again polymerization is takento virtually complete conversion of the monomers. In thispolymerization, the monomers added in the second stage of the processare added onto the active living chain ends of the previously formedmonomer segments A¹, but new chains of living polymers are also formedby the fresh initiator added. Because of the different copolymerizationparameters, the conjugated dienes polymerize substantially more rapidlythan the monovinyl-aromatic compounds, so that after addition of themonomer mixture in the second stage of the process, it is firstpredominantly the conjugated dienes which undergo polymerization, andonly occasional polymerized units of the monovinyl-aromatic compoundsare formed. Only toward the end of the diene polymerization, ie. whenalmost all the conjugated diene has polymerized, does the polymerizationof the monovinyl-aromatic compounds commence to a significant degree, sothat the predominant proportion -- as a rule more than 70 percent byweight, and in most cases up to 80 percent by weight -- of themonovinyl-aromatic compounds contained in the monomer mixture onlypolymerizes after the conjugated diene has been consumed.

Accordingly, in the second stage of the process an elastomeric polymersegment B, based on the conjugated dienes, is first formed, this being acopolymer of a predominant proportion of the conjugation diene withsmall amounts of the monovinyl-aromatic compound, after which anon-elastomeric polymer segment A² is formed, which is made up of themonovinyl-aromatic compounds only. Since the proportion of themonovinyl-aromatic compounds progressively increases toward the end ofthe polymer segment B and the proportion of the conjugated dienecorrespondingly decreases, the transition between the polymer segments Band A² formed is not sharp and instread occurs gradually; this istherefore frequently described as a blurred transition between thesegments. This fact is taken into account, in the general formula forthe branched block copolymers of the invention, by the use of the symbol→.

After complete polymerization of the monomer mixture in the second stageof the process, the reaction solution thus contains a mixture of livinglinear block copolymers of the type (A¹ --B→A²)--Li and (B→A²)--Li, eachwith reactive lithium-carbon bonds at the free end of the polymersegments A². The ratio of the two types of block copolymers in thereaction solution corresponds to the ratio of the amounts of initiatorin the first and second stages of the process.

The mixture of these two types of active living linear block copolymersis then reacted in a further stage of the process, in which is added apolyfunctional reactive compound to act as the coupling agent. Thepolyfunctional coupling agent used should be at least trifunctional, ie.it should be capable of reacting with three or more of the active livingblock copolymer chains, at the terminal lithium-carbon bonds of these,to form a chemical bond, so that a single coupled and accordinglybranched block copolymer is formed. The coupling of lithium-terminatedliving polymers with polyfunctional coupling agents is known in the artand disclosed, for example, in the publications cited initially,especially British Pat. No. 985,614.

Examples of suitable coupling agents for the manufacture of the branchedblock copolymers of the invention are polyepoxides, eg. epoxidizedlinseed oil, polyisocyanates, eg. benzo-1,2,4-triisocyanate,polyketones, eg. 1,3,6-hexanetrione or 1,4,9,10-anthracenetetrone,polyanhydrides, eg. the dianhydride of pyromellitic acid, orpolyhalides. Dicarboxylic acid esters, eg. diethyl adipate or the like,can equally be used as coupling agents. A further preferred group ofcoupling agents comprises the silicon halides, especially silicontetrachloride, silicon tetrabromide, trichloroethylsilane or1,2-bis(methyldichlorosilyl)-ethane. Further coupling agents which canbe employed are polyvinyl-aromatics, especially divinylbenzene, asdescribed, eg., in U.S. Pat. No. 3,280,084. In this case, somedivinylbenzene units add on, producing crosslinking and forming abranching center, through which the preformed polymer blocks are bondedto one another.

The nature of the polyfunctional coupling agent used is not criticalprovided it does not significantly detract from the desired propertiesof the end product. The use of a trifunctional or tetrafunctionalcoupling agent of the above type, or of divinylbenzene, is preferred. Ingeneral, the polyfunctional coupling agent is added to the reactionsolution in amounts equivalent to the total amount of the "living"polymer blocks, ie. equivalent to the number of active lithium-carbonbonds in the preformed linear block copolymers. The reaction of theliving linear block copolymers with the coupling agent is preferablycarried out under the same reaction conditions as the precedingpolymerization of the monomers. The resulting branched block copolymersare isolated from the reaction solution by conventional methods, eg. byprecipitating the polymer from the reaction solution, and filtering itoff.

If desired, the branched block copolymer can be hydrogenated followingthe coupling reaction and, advantageously, before isolating the productfrom the reaction solution. The hydrogenation may be carried outselectively or non-selectively and is normally effected with the aid ofmolecular hydrogen and catalysts based on metals, or salts of metals, ofgroup 8 of the periodic table. The hydrogenation can be carried out in ahomogeneous phase with catalysts based on salts, especially thecarboxylates, alkoxides or enolates of cobalt, nickel or iron, whichhave been reduced with metal alkyls, especially aluminum alkyls, asdisclosed, for example, in U.S. Pat. No. 3,113,986, German PublishedApplication DAS 1,222,260 or German Laid-Open Application DOS 2,013,263.In these reactions, the olefinic double bonds are hydrogenated undermild conditions at hydrogen pressures of from 1 to 100 bars, and at from25° to 150° C. The hydrogenation can also be carried out in aheterogeneous phase, with metallic nickel or metals of the platinumgroup as catalysts, at hydrogen pressures of from 20 to 300 bars and atfrom 40 to 300 bars and at from 40° to 300° C (for example, by themethod of German Published Application DAS 1,106,961 or German Laid-OpenApplication DOS 1,595,345). In this reaction, not only the olefinicdouble bonds but also the aromatic double bonds are hydrogenated. If thehydrogenation is carried out in solution, it is advantageously effectedin the same solvent as the preceding polymerization. The branched blockcopolymer may be hydrogenated partially or completely. If ahydrogenation is carried out, it is preferred selectively to hydrogenatethe olefinic double bonds of the polymer, so that the hydrogenatedbranched copolymers obtained preferably only contain less than 10% andespecially less than 3%, of olefinic double bonds. The hydrogenation ispreferably carried out on branched block copolymers which have beenmanufactured in the presence of small amounts of ethers during thepolymerization.

The process of manufacture decides the composition and structure of thebranched block copolymers of the invention. If, for example, atetrafunctional coupling agent is used and the ratio, in the fullypolymerized reaction solution from the second stage of the process, ofthe two types of block copolymers which form the branches, namely theratio of (A¹ --B→A²)--Li to (B→A²)--Li, is, for example, 1:1 or 1:3, theresulting branched block copolymer will on average (most probablestructure) possess a structure of the formula

    (A.sup.1 --B→A.sup.2).sub.2 --X--(A.sup.2 ←B).sub.2

or

    (A.sup.1 --B→A.sup.2).sub.1 --X--(A.sup.2 ←B).sub.3.

in the case of a trifunctional coupling agent and a ratio of the twotypes of branches, namely (A¹ --B→A²)--Li to (B←A²)--Li, of 1:2, themost probable average structure of the branched block copolymer is

    (A.sup.1 --B→A.sup.2).sub.1 --X--(A.sup.2 ←B).sub.2 ;

in each of the formulae, X is the radical of the coupling agent.

In general, the most probable average structure of the branched blockcopolymers manufactured according to the invention corresponds to thegeneral formula

    (A.sup.1 --B→A.sup.2).sub.n --X--(A.sup.2 ←B).sub.m

where m and n are integers, and the sum of n and m is equal to thepolyfunctionality of the coupling component and is thus at least 3, ingeneral from 3 to 10 and preferably 3 or 4. m is equal to or greaterthan n. The non-elastomeric polymer segment A¹, which contains from 50to 80 percent by weight, preferably from 60 to 78 percent by weight, ofthe total monovinyl-aromatic compound, employed for the manufacture ofthe branched block copolymer, as polymerized units, preferably consistsonly of the monovinylaromatic compounds and is thus in particular ahomopolystyrene segment. Its molecular weight depends particularly onthe envisaged end use of the final product and is preferably from 50,000to 250,000. As stated, the elastomeric polymer segment B is a copolymerblock consisting essentially of the conjugated diene with a smallproportion of monovinyl-aromatic compound, and in particular theolefinic double bonds can be selectively hydrogenated. The proportion ofmonovinyl-aromatic compound in the polymer segment B is in general lessthan about 30 percent by weight and in particular less than about 20percent by weight, based on the amount of vinyl-aromatic not containedin the polymer segment A¹. The non-elastomeric polymer segments A², linethe polymer segment A¹, preferably are built up of themonovinyl-aromatic compound alone, and in particular arehomopolystyrene. The molecular weight of the polymer blocks (B→A²) ispreferably from 10,000 to 100,000.

The branched block copolymers of the invention possess high transparencyand clarity and good mechanical properties, and in particular aresuperior, in respect of impact strength and elongation at break, to theconventional products described in German Laid-Open Application DOS1,959,922. This was not foreseeable, and was all the more surprisingsince, according to the teaching of DOS 1,959,922, all non-elastomericpolymer segments must be in terminal positions if satisfactorymechanical properties are to be achieved. Hydrogenation can inparticular improve the aging resistance of the products, though it mayresult in some reduction in their transparency. The branched blockcopolymers of the invention can easily be processed by the conventionalprocessing methods for thermoplastics, eg. extrusion, deep-drawing orinjection molding, and may be used, for example, for the manufacture ofpackaging.

The Examples which follow illustrate the invention. The viscositynumber, measured in 0.5% strength solution in toluene at 25° C, isquoted as a measure of the molecular weight. The impact strength a_(n)and notched impact strength a_(k) were determined on a molded specimenaccording to DIN 53,453. The yield stress Y, tensile strength Z andelongation at break D were measured on a compression-moldeddumbbell-shaped bar according to DIN 53,455.

EXAMPLE 1

2 kg of toluene and 250 g of styrene were titrated with n-butyl-lithium,in a 6 l pressure kettle under an inert gas atmosphere, with exclusionof moisture, until polymerization commenced. 2.25 mmoles ofn-butyl-lithium (as a solution in n-hexane) were then added and themixture was polymerized at 50° C for about 1.5 hours, until the styrenewas virtually completely converted. The resulting polystyrene segmentshad a viscosity number of 47.2 (cm³ /g). A further 6.75 mmoles ofn-butyl-lithium (as a solution in n-hexane) were added to the solutionof polystyryl-lithium, and a mixture of 125 g of styrene and 125 g ofbutadiene was then added to the reaction solution. The temperature waskept at from 50° to 55° C. After about 3 hours, polymerization wasvirtually complete. 2.25 mmoles of silicon tetrachloride were then addedas the coupling agent and the reaction solution was kept for 15 hours atroom temperature. The polymer was then precipitated from the solution byadding methanol, filtered off and dried.

The branched block copolymer obtained had an average approximatestructure of [polystyrene-poly(butadiene/styrene)→polystyrene]₁-Si-[polystyrene ←poly(butadiene/styrene)]₃ and a viscosity number of76.8 (cm³ /g). Its mechanical properties are shown in the table below.

EXAMPLE 2

The procedure followed was as described in Example 1, but in this case2.4 kg of toluene and 350 g of styrene were initially introduced intothe reactor. After titrating the reaction solution with n-butyl-lithium,4.66 mmoles of n-butyl-lithium were added to initiate thepolymerization, which was carried out at 50° C until the styrene wascompletely converted. The resulting polystryene had a viscosity numberof 37.1 (cm³ /g). A further 4.66 mmoles of n-butyl-lithium were thenadded, followed by a mixture of 106 g of styrene and 170 g of butadiene.After completion of the polymerization, coupling was carried out with23.3 mmoles of silicon tetrachloride. The viscosity number of the endproduct was 99.9 (cm³ /g). The approximate structure of the branchedblock copolymer was [polystyrene-poly(butadiene/styrene)→polystyrene]₂-Si-[polystyrene-poly(butadiene/styrene)]₂. The mechanical propertiesare summarized in the table.

COMPARATIVE EXAMPLE (according to German Laid-Open Application DOS1,959,922)

2.7 kg of cyclohexane and 600 g of styrene were titrated withsec.-butyl-lithium in a 6 l pressure kettle under an inert gasatmosphere and then polymerized for 30 minutes with 0.33 g ofsec.-butyl-lithium. The initial temperature was 54° C. 0.22 kg ofcyclohexane, 0.9 g of sec-butyl-lithium and 225 g of styrene were addedto the reaction solution at 71° C, polymerization was carried out forone hour, and 250 g of butadiene were then polymerized onto the productin the course of 1 hour at about 74° C. Finally, coupling was carriedout with 10 ml of Epoxyl 9-5 in 150 ml of toluene. The product wasprecipitated from isopropanol. The viscosity number was 91.9 (cm³ /g).

                  TABLE                                                           ______________________________________                                               a.sub.n                                                                              a.sub.k                                                                (cm . kg/                                                                            (cm . kg/                                                                              Y        Z      D                                             cm.sup.2)                                                                            cm.sup.2)                                                                              (kp/cm.sup.2)                                                                          (kp/cm.sup.2)                                                                        (%)                                    ______________________________________                                        Example 1                                                                              33.5     16.8     290    175    410                                  Example 2                                                                              30%      2.5      214    269    431                                           fracture                                                             Comparative                                                                   Example  15.1     5.4      170    190     91                                  ______________________________________                                    

We claim:
 1. Branched block copolymers of from 60 to 95 percent by weight of a monovinyl-aromatic compound and from 40 to 5 percent by weight of a conjugated diene of 4 to 8 carbon atoms, which have an average structure of the general formula

    (A.sup.1 --B→A.sup.2).sub.n --X--(A.sup.2 ←B).sub.m

where A¹ and A² are non-elastomeric polymer segments based on the monovinyl-aromatic compound and the B's are elastomeric polymer segments based on the conjugated diene, n and m are numbers, m being equal to or greater than n and the sum of m and n being at least 3, and X is the radical of the polyfunctional coupling agent by means of which the linear polymer blocks (A¹ --B→A²) and (B→A²), which form the branches, are chemically bonded to one another at the polymer segments A², with the provisos that the polymer segment or segments A¹ contains or contain from 50 to 80 percent by weight of the total monovinyl-aromatic compound of the branched block copolymer, as copolymerized units, the transition between the polymer segments A¹ and B is sharp and the transition between the polymer segments B and A² is gradual.
 2. Branched block copolymers as set forth in claim 1, which have a weight-average molecular weight of from 100,000 to 1,000,000.
 3. Branched block copolymers as set forth in claim 1, which are partially or completely hydrogenated.
 4. A process for the manufacture of branched block copolymers of claim 1 which comprises: polymerizing in a first stage of the process, from 50 to 80 percent by weight of the total amount of monovinyl-aromatic compound in an inert solvent, in the presence of a relatively small amount of a monolithium-hydrocarbon as the initiator until conversion is virtually complete, thereafter, in a second stage of the process, adding to the reaction solution a further amount of monolithium-hydrocarbon, which is equal to or greater than the amount of initiator originally employed, followed by the addition of a mixture of the remaining monovinyl-aromatic compound and the conjugated diene, and again carrying out the polymerization until the monomers have been virtually completely converted, thereafter subjecting the mixture of the resulting linear block copolymers with active terminal lithium-carbon bonds to a coupling reaction, by adding a polyfunctional coupling agent, to form a branched block copolymer and finally isolating the branched block copolymer from the reaction solution.
 5. A branched block copolymer as set forth in claim 1, wherein the monovinyl-aromatic compound is styrene, styrene alkylated in the side chain and nuclear substituted styrene.
 6. A branched block copolymer as set forth in claim 1, wherein the conjugated diene is butadiene, isoprene and 2,3-dimethylbutadiene.
 7. A branched block copolymer as set forth in claim 1, wherein the polyfunctional coupling agent is a polyepoxide, polyisocyanate, polyketone, polyanhydride, polyhalide, dicarboxylic acid ester, silicon halide or polyvinyl-aromatic.
 8. A branched block copolymer as set forth in claim 1, wherein the sum of n + m is from 3 to
 10. 9. A process as set forth in claim 4, wherein the amount of initiator in the first stage is from 0.2 to 10 moles per mole of the monovinyl-aromatic compound.
 10. A process as set forth in claim 4, wherein the amount of initiator in the second stage is from 1 to 15 times the amount of initiator added in the first stage. 